In a conventional system for driving engine accessories from an engine, the accessories are driven through driven pulleys by a drive belt which is received on a drive pulley connected to the engine. Thus the speed ratio between the engine (or the drive pulley driven therefrom) and any particular accessory has been constant.
It has been recognized for some time that it is not desirable to drive an accessory at a constant ratio to the speed of the engine. Such a practice results generally in too low a speed for the accessory at low engine speeds and too high a speed for the accessory at high engine speeds. In other words, it has heretofore been realized that a variable speed ratio between the engine and the accessories to maintain the accessory speed more nearly constant over the range of the engine speed would be desirable.
Many different constructions have been proposed to achieve this goal. One proposed solution, disclosed in the copending United States patent application of L. J. Stephanoff, Ser. No. 422,250, filed Dec. 6, 1973, now abandoned, and assigned to the same assignee as the present invention, utilizes a centrifugally operated variable diameter pulley which is driven by the engine. A first belt, trained around the drive pulley and an idler pulley, is provided solely to make good driving contact with the drive pulley. A second belt is trained over the first belt at the drive pulley, in good driving engagement with the first belt, and is received on pulleys of the accessories to be driven.